A stable and adjustable source of extremely low frequency waveforms has been needed in the past for the testing and development of various electronic components such as amplifiers, filters, spectrum analyzers and the like. Recent interest in amplifier a filter circuits having known and specified response characteristics down to very low frequencies in the region of 0.01 Hz and below have resulted in the need for a highly stable signal source of such frequencies variable both in frequency and amplitude.
Conventional oscillatory circuits of the LC or RC type (e.g., Wien Bridge circuits) utilize AC feed-back to maintain oscillation and DC feedback from an averaged output for output amplitude stabilization. Therefore, several cycles are necessary for frequency stabilization and many cycles are required for amplitude stabilization whenever an operational change is made. While such oscillators have been constructed to operate at frequencies below 0.01 Hz, the stabilization waiting times become very long and generally impractical. At these very low frequencies, waveform synthesis becomes a more practical solution for a signal source. Digital sine and cosine synthesis techniques have been used in the past for this purpose, but at the present time are undesirably expensive. Therefore, a need has developed for a more inexpensive AC waveform synthesis technique which inexpensively produces a stable waveform.